Omega phase in materials

Sikka, S. K.; Vohra, Yogesh K.; Chidambaram, R.

doi:10.1016/0079-6425(82)90002-0

Cited by 745 publications

(413 citation statements)

References 87 publications

Supporting

Mentioning

382

Contrasting

Unclassified

Order By: Relevance

“…Free energy calculations of ␣ and within the quasiharmonic approximation using a tight-binding model show that the ␣ phase is stabilized at ambient temperature by phonon entropy. 37 Our ab initio calculations show in agreement with experiments 34 and earlier theoretical work 27,28 the cohesive energy of to be 5 meV/atom lower than ␣. For Zr the two phases are calculated to be degenerate.…”

Section: Ti-zr-ni Crystalline Phase Diagramsupporting

confidence: 79%

“…36 For both Ti and Zr the crystal structures at 0 K have not been determined experimentally. However, extrapolation of the ␣ − phase boundary 34 in Ti indicates as the ground-state phase. Free energy calculations of ␣ and within the quasiharmonic approximation using a tight-binding model show that the ␣ phase is stabilized at ambient temperature by phonon entropy.…”

Section: Ti-zr-ni Crystalline Phase Diagrammentioning

confidence: 99%

See 1 more Smart Citation

Ab initioTi-Zr-Ni phase diagram predicts stability of icosahedral TiZrNi quasicrystals

et al. 2005

View full text Add to dashboard Cite

The ab initio phase diagram determines the energetic stability of the icosahedral TiZrNi quasicrystal. The complete ab initio zero-temperature ternary phase diagram is constructed from the calculated energies of the elemental, binary and ternary Ti-Zr-Ni phases. For this, the icosahedral i-TiZrNi quasicrystal is approximated by periodic structures of up to 123 atoms/unit cell, based on a decorated-tiling model ͓R. G. Hennig, K. F. Kelton, A. E. Carlsson, and C. L. Henley, Phys. Rev. B 67, 134202 ͑2003͔͒. The approximant structures containing the 45-atom Bergman cluster are nearly degenerate in energy, and are all energetically stable against the competing phases. It is concluded that i-TiZrNi is a ground-state quasicrystal, as it is experimentally the low-temperature phase for its composition.

show abstract

Section: Ti-zr-ni Crystalline Phase Diagramsupporting

confidence: 79%

Section: Ti-zr-ni Crystalline Phase Diagrammentioning

confidence: 99%

Ab initioTi-Zr-Ni phase diagram predicts stability of icosahedral TiZrNi quasicrystals

et al. 2005

View full text Add to dashboard Cite

show abstract

“…It is this pattern of displacement that is directly involved in the bcc to omega (ω) phase transition (see below). The transition behaviour to the hcp and ω phases is very similar to that observed in Group IV metals in the bcc phase (Petry et al, 1991); however, it is interesting to note that Hf, Zr abd Ti do not display an initial zone centre instability to the fcc phase, but rather, as has been observed, transform first to the ω phase (Sikka et al, 1982).…”

Section: Introductionmentioning

confidence: 68%

The stability of bcc-Fe at high pressures and temperatures with respect to tetragonal strain

Vočadlo

Wood

Gillan

et al. 2008

Physics of the Earth and Planetary Interiors

View full text Add to dashboard Cite

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractThe phase that iron adopts at the conditions of the Earth's inner core is still unknown. The two primary candidates are the hexagonal-close-packed (hcp) structure and the bodycentred-cubic (bcc) structure polymorphs. Until recently, the former was favoured, but it now seems possible that bcc iron could be present. A remaining uncertainty regarding the latter phase is whether or not bcc iron is stable with respect to tetragonal strain under core conditions. In this paper, therefore, we present the results of high precision ab initio free energy calculations at core pressures and temperatures performed on bcc iron as a function of tetragonal strain. Within the uncertainties of the calculations, direct comparison of free energy values suggests that bcc may be unstable with respect to tetragonal strain at 5500 K; this is confirmed when the associated stresses are taken into account. However, at 6000 K, the results indicate that the bcc phase becomes more stable, although it is unclear as to whether complete stability has been achieved. Nevertheless, it remains distinctly possible that the addition of light elements could stabilise this structure convincingly. Therefore, bcc-Fe cannot be ruled out as a candidate inner core phase.

show abstract

“…It is well known that the hex-ω phase appears in the equilibrium high-pressure phase diagrams of the group-IVB metals Ti, Zr, and Hf [45]. A variant of the hex-ω structure, the trigonal omega (trig-ω) structure, which contains a second internal parameter z, also appears in the phase diagrams of group-IVB alloys with other central transition metals, including the group-VB elements V, Nb and Ta.…”

Section: B Hexagonal Omega Phasementioning

confidence: 99%

Polymorphism and melt in high-pressure tantalum

2012

View full text Add to dashboard Cite

Recent small-cell (< 150-atom) quantum molecular dynamics (QMD) simulations for Ta based on density functional theory (DFT) have predicted a hexagonal omega (hex-ω) phase more stable than the normal bcc phase at high temperature (T) and pressure (P) above 70 GPa [Burakovsky et al., Phys. Rev. Lett. 104, 255702 (2010)]. Here we examine possible high-T , P polymorphism in Ta with complementary DFT-based model generalized pseudopotential theory (MGPT) multi-ion interatomic potentials, which allow accurate treatment of much larger system sizes (up to ~ 80000 atoms). We focus on candidate bcc, A15, fcc, hcp, and hex-ω phases for the high-T , P phase diagram to 420 GPa, studying the mechanical and relative thermodynamic stability of these phases for both small and large computational cells. Our MGPT potentials fully capture the T = 0 DFT energetics of these phases, while MGPT-MD simulations demonstrate that the higher-energy fcc, hcp and hex-ω structures are only mechanically stabilized at high temperature by large, size-dependent, anharmonic vibrational effects, with the stability of the hex-ω phase also being found to be a sensitive function of its c / a ratio. Both twophase and Z-method melting techniques have been used in MGPT-MD simulations to determine relative phase stability and its size dependence. In the large-cell limit, the twophase method yields accurate equilibrium melt curves for all five phases, with bcc producing the highest melt temperatures at all pressures and hence being the most stable phase of those considered. The two-phase bcc melt curve is also in good agreement with dynamic experimental data as well as with the MGPT melt curve calculated from bcc and 2 liquid free energies. In contrast, we find that the Z method produces only an upper bound to the equilibrium melt curve in the large-cell limit. For the bcc and hex-ω structures, however, this is a close upper bound within 5% of the two-phase results, although for the A15, fcc, and hcp structures, the Z-melt curves are 25-35% higher in temperature than the two-phase results. Nonetheless, the Z method has allowed us to study melt size effects in detail. We find these effects to be either small or modest for the cubic bcc, A15, and fcc structures, but to have a large impact on the hexagonal hcp and hex-ω melt curves, which are dramatically pushed above that of bcc for simulation cells less than 150 atoms. The melt size effects are driven by and closely correlated with similar size effects on the mechanical stability and the vibrational anharmonicity. We further show that for the same simulation cell sizes and choice of c / a ratio, the MGPT-MD bcc and hex-ω melt curves are in good agreement with the QMD results, so the QMD prediction is confirmed in the small-cell limit. But in the large-cell limit, the MGPT-MD hex-ω melt curve is always lowered below that of bcc for any choice of c / a , so bcc is the most stable phase.We conclude that for the non-bcc Ta phases studied, one requires simulation cells of at least 250-500 atoms to be free ...

show abstract

Omega phase in materials

Cited by 745 publications

References 87 publications

Ab initioTi-Zr-Ni phase diagram predicts stability of icosahedral TiZrNi quasicrystals

Ab initioTi-Zr-Ni phase diagram predicts stability of icosahedral TiZrNi quasicrystals

The stability of bcc-Fe at high pressures and temperatures with respect to tetragonal strain

Polymorphism and melt in high-pressure tantalum

Contact Info

Product

Resources

About